Introduction

The middle Cretaceous Colorado/Alberta Group consists predominantly of mudstone
interspersed with relatively thin sandstone and conglomerate beds. Other minor lithotypes
include shaly chalk, chalky limestone, bentonite, pelecypod coquinas, horizons of
fish debris, nodular phosphorite, and siderite, calcite and pyrite concretions. The
Colorado Group is of significant economic importance in that it contains about 14
percent of the total Western Canada hydrocarbon reserves and about 80 percent of
the reserves within the Middle Jurassic to Cretaceous foreland basin succession (Podruski
et al., 1988; Porter, 1992). The largest and historically oldest gas pool in Canada
occurs within the Medicine Hat Sandstone of the Colorado Group; this pool was discovered
in 1890 during early exploration for coal resources.

This chapter presents an overview of the dominantly shaly Colorado and Alberta
groups, hereinafter referred to as the Colorado Group. Other following chapters describe
in detail the coarser clastic wedges represented by the Viking, Dunvegan and Cardium
formations (Chapters 21, 22 and 23, respectively).

Previous Work

Earlier regional syntheses of the Colorado Group are in Caldwell (1984) and Williams
and Burke (1964). Detailed lithostratigraphic and micropaleontological aspects of
the Colorado Group in the Manitoba Escarpment and adjacent subsurface were described
in McNeil and Caldwell (1981) and McNeil (1984). In Saskatchewan, detailed lithological
descriptions were carried out by Simpson (1982). Stott (1963, 1967, 1982) meticulously
mapped the foothills of Alberta and British Columbia, providing a comprehensive lithostratigraphy
of the Upper Cretaceous strata. This lithostratigraphic framework has served as an
excellent starting point for more detailed studies. The biostratigraphic zonation
of Cretaceous strata in Western Canada was synthesized by Caldwell et al. (1978).

Summary lithological and stratigraphic descriptions of the units making up the
Colorado Group across the basin are in Glass (1990). Highly generalized maps illustrating
the extent of marine inundation were presented by Williams and Stelck (1975). Detailed
paleogeographic maps illustrating the evolution of the Western Canada Sedimentary
Basin are presented in Leckie and Smith (1992).

Geological Framework

The Albian to Santonian Colorado Group was deposited within the Western Canada
Foreland Basin during an approximately 25 to 30 million year period. Global sea level
was high during this time, with specific sea-level maxima in the Late Albian, Early
Turonian and Middle Santonian (Caldwell, 1984; Haq et al., 1987). Deposition at this
time was also coincident with a regional tectonic downflexing of the North American
craton (Lambeck et al., 1987). The major marine inundations were separated by four
major regressive pulses represented by the Peace River-Viking, Dunvegan, Cardium-Bad
Heart and Milk River formations. During the highstands, warm Tethyan water from the
Gulf of Mexico mixed with the cooler boreal water extending south from the Arctic
to form a shallow epeiric seaway.

The Colorado Group contains several sandstone and conglomerate units, some of
which are prolific hydrocarbon producers (Table 20.1).
These include, in ascending order, the Basal Colorado Sandstone, Spinney Hill Sandstone,
Viking Formation, St. Walburg Sandstone, Barons Sandstone, Dunvegan Formation, sandstones
of the lower Kaskapau Formation (Doe Creek Member), sandstones of the Second White
Speckled Shale (the Phillips Sandstone), Cardium Formation, the Medicine Hat Sandstone
and the Alderson Member of the Lea Park Formation (Fig. 20.1).
Within the Colorado Group, the First and Second White Speckled Shales, the Fish Scales
Zone, and shale at the base of the Shaftesbury Formation are more radioactive than
overlying and underlying shales, have high total organic carbon contents, and have
considerable hydrocarbon generating potential. An interval such as the Second White
Speckled Shale is potentially both a source and a reservoir rock for hydrocarbons.

The Colorado Group thins eastward from about 700 m in southwestern Alberta to
200 m in the Manitoba Escarpment (Fig. 20.2). In northwest
Alberta, the Colorado Group exceeds 1500 m in thickness where it overlies the Peace
River Arch, which was subsiding during much of the Cretaceous. Regional cross sections
constructed across the basin show the eastward thinning of the Colorado Group away
from the Cordillera, with maximum thickening occurring in the northwest. The distribution
of the Harmon, Cadotte and Paddy members is restricted to the general vicinity of
the Peace River Arch where the Joli Fou Formation is absent.

Major structural elements affecting Colorado Group deposition in the basin are
represented in the structure maps constructed on the Base of Fish Scales Zone
(Fig. 20.3) and the top of the Milk River Formation
(Fig. 20.4).
The major elements identified on Figure 20.3 are similar to those identified by Williams and Burk (1964).

Positive structural elements include the Bow Island Arch (also referred to as
the Sweetgrass Arch) in southeastern Alberta, and the Bowdoin Dome and Swift Current
Platform in southern Saskatchewan. The Bow Island Arch separates the Alberta Basin
from the Williston Basin.

Negative structural elements include the Williston Basin and its northern extension,
the Moose Jaw Syncline in southern Saskatchewan and the Eastend Syncline in southwestern
Saskatchewan. In southeastern Saskatchewan and southwestern Manitoba, the influence
of the Williston Basin is evident in structural lows centred southeast of Regina.
In northwestern Alberta and northeastern British Columbia, the Peace River Arch began
to subside, with accompanying block faulting, during the Mississippian. It remained
a topographic low during the Early Cretaceous and through to the Late Cretaceous
(Cant, 1988). The Peace River Arch may have become a subtle high again by at least
the late Turonian-early Santonian. Isolated stratigraphic and structural anomalies
are attributed to local block-faulting over the Peace River Arch during the deposition
of the Colorado Group.

The influence of the dissolution of Devonian-aged salts has been described elsewhere
in this atlas (Wright, et al., this volume, Chapter 3). Salt solution during or prior
to deposition of the Colorado Group sediments created local anomalous thickening.
Salt solution subsequent to deposition of the Colorado Group has resulted in depressions
on structural surfaces.

Regional cross sections from the top of the Mannville Group to the top of the
Milk River Formation are illustrated in Figures 20.5, 20.6, 20.7, 20.8, 20.9, 20.10. The datum for all
these sections is the Base of Fish Scales Zone, which occurs across most of the basin,
although it is sometimes difficult to identify in parts of Saskatchewan. Some difficulties
with identification and correlation of the Second White Speckled Shale are evident
in Figures 20.5, 20.9 and 20.10 because of changes in log character across the
basin. Cross section H-H' (Fig. 20.9) is oriented along
the axis of the western Alberta foreland basin and highlights the geometry of the
Dunvegan clastic wedge. The northwestern end of the cross section stops where the
units come to the surface in the Peace River valley.

The suite of reference logs (Fig. 20.18 left pane, 20.18 right pane) illustrates
the variability of the different components of the Colorado Group across the basin
as well as the markers used for correlation.

Stratigraphy

The stratigraphic terminology of the Colorado Group includes several informal
but generally accepted names such as the First and Second White Speckled Shales and
the Fish Scales Zone. The terminology, summarized in Figure
20.1, varies from province to province and between regions in the provinces.
Since the Colorado Group is a predominantly marine shale succession, it has been
well constrained biostratigraphically using foraminifera (Caldwell et al., 1978;
McNeil and Caldwell, 1981), ammonites (Jeletzky, 1971) and microflora (Singh, 1983).

The Colorado Group was informally subdivided by Rudkin (1964) into upper and lower
subgroups separated by the Base of Fish Scales Zone. In Saskatchewan, where the Fish
Scales Zone is not always evident, Simpson (1982) subdivided the Colorado Group at
the lower of two calcareous speckled shales (the Second White Speckled Shale). Neither
subdivision is used herein.

The term Alberta Group is restricted to those strata south of the Smoky River
in the southern Alberta Foothills and includes the Blackstone, Cardium and Wapiabi
formations (Fig. 20.1). In northwestern Alberta and northeastern
British Columbia, correlative strata include the Shaftesbury and Dunvegan formations
and that part of the Smoky Group comprising the Kaskapau, Cardium, Muskiki, Marshybank,
Badheart and lowermost Puskwaskau formations. In the subsurface of Alberta and Saskatchewan,
the Colorado Group is used. In Manitoba, strata equivalent to the Alberta and Colorado
groups are called the Ashville, Favel, Morden and Niobrara formations (McNeil and
Caldwell, 1981). Within the Ashville Formation, only the Newcastle Member consists
predominantly of sandstone. In northernmost British Columbia, correlative strata
are the upper Lepine, Sikanni, Sully, Dunvegan and lower Kotaneelee formations.

Stratigraphic History

Basal Contact and Units

The base of the Colorado Group is represented by a basin-wide unconformity. The
Colorado Group overlies the Blairmore Group in western Alberta, the Mannville Group
in the subsurface of southern Alberta and Saskatchewan, the Cadotte Member of the
Fort St. John Group in northeastern British Columbia and northwestern Alberta, and
the Swan River Formation in Manitoba (Figs. 20.1 and 20.5, 20.6, 20.7, 20.8, 20.9, 20.10). The basal contact of the Colorado Group with the Mannville Formation
is generally represented by a thin conglomerate layer containing chert or intraformational
shale clasts. In the Manitoba Escarpment, the unconformity likely occurs within the
Swan River Formation. The upper part of the Swan River is marine (Pense equivalent)
and the lower part is terrestrial (Cantuar equivalent).

The lowermost formal lithostratigraphic unit of the Colorado Group is represented
by the Spinney Hill Sandstone in eastern and central Saskatchewan, the Basal Colorado
Sandstone in southeastern Alberta and shale of the Joli Fou Formation in most other
areas (Figs. 20.1 and 20.5, 20.6, 20.7, 20.8, 20.9, 20.10).

The Basal Colorado Sandstone, in southeastern Alberta, southern Saskatchewan,
northern Montana and North Dakota, is a thin (less than m thick), sheet-like unit
underlying the Joli Fou marine shales and overlying the non-marine Mannville Group
(Banerjee, 1989). In the Cessford area (Tp 22-26 R 10-16W4), the Basal Colorado Sandstone
(also called Cessford Sand) occurs as a northwest-trending sandstone body, 20 km
wide, 6 to 8 m thick, and situated in a paleotopographic low. In Saskatchewan, the
approximately correlative Spinney Hill Formation is a grayish to yellowish green,
very fine- to coarse-grained sandstone with noncalcareous shale interbeds and glauconite.
Other lithotypes include intraformational conglomerates, clasts of nodular phosphorite,
pelecypod coquinas and concretions of siderite and pyrite. The formation is up to
36.6 m thick and occurs as a sandstone body up to 60 km wide in central Saskatchewan
(Simpson, 1982).

In northwest Alberta and northeast British Columbia, the basal contact of Colorado
Group sediments is represented by a major erosion surface between the Paddy and Cadotte
members of the Peace River Formation (Figs. 20.9, 20.10). This unconformity has bevelled the underlying
Cadotte and Harmon members toward the east and south. Westward, in the Rocky Mountain Foothills
of northeastern British Columbia, the unconformity approximately corresponds to the
position of the lowermost paleosols within the Boulder Creek Formation (Leckie et
al., 1989).

Joli Fou Formation

The Upper Albian Joli Fou Formation (Figs. 20.1 and 20.5, 20.6, 20.7, 20.8, 20.9, 20.10) is a dark gray, noncalcareous marine shale with a small proportion
of interbedded fine- to medium-grained sandstone. Minor amounts of nodular phosphorite,
bentonite, pelecypod coquinas and concretions of siderite, calcite and pyrite also
occur. The Joli Fou Formation is extremely widespread, and is distributed throughout
the subsurface, except in parts of northwestern Alberta and northeastern British
Columbia (Figs. 20.9, 20.10)
where it has been apparently truncated by an unconformity at the base of the Paddy
Member. In the Manitoba Escarpment, the Joli Fou Formation is called the Skull Creek
Member. In western and central regions, the Joli Fou Formation disconformably overlies
the Mannville Group where the Basal Colorado/Spinney Hill are absent, and underlies
the Viking Formation, disconformably in places. In eastern Saskatchewan, the Joli
Fou Formation is underlain by the Swan River Sandstones, and in parts of Manitoba,
it locally overlies Jurassic-aged sediments. The Joli Fou passes laterally southwestward
into the Bow Island Formation (Fig. 20.7) and is not
known to occur in outcrop of the Rocky Mountain Foothills. It is exposed and has
a type section along the Athabasca River (Wickenden, 1949). Fauna within the Joli
Fou shale suggest that the marine seaway extended from central Alberta to the Gulf
of Mexico.

Viking Formation and Equivalent Units

The Albian Viking Formation, which is found in central and southern Alberta and
Saskatchewan, was first named by Slipper (1918) to describe a gas-bearing sandstone
encased in shale in east-central Alberta. Correlative units (Fig.
20.1) include the Bow Island Formation in southwestern Alberta (Fig.
20.7), the Newcastle Sand in Manitoba (Fig. 20.8),
the Pelican Formation in northeastern Alberta, the Paddy Member (Peace River Formation)
in the northwestern Alberta and northeastern British Columbia subsurface (Fig.
20.9), part of the Walton Member of the Boulder Creek Formation in northeastern
British Columbia, and the Flotten Lake Sandstone in Saskatchewan. The Bow Island
Formation thins eastward and northward from the Rocky Mountain Foothills, where it
crops out as the Mill Creek Formation.

The Viking Formation and equivalent units are an eastward-thinning wedge of coarse
clastic detritus, which extends from British Columbia to Saskatchewan. In central
Alberta, the thickness of the Viking Formation is 15 to 30 m; southward, it thickens
to more than 75 m, and eastward, it decreases until the unit pinches out in central
and eastern Saskatchewan. The formation is described in detail by Reinson et al.
(this volume, Chapter 21). It consists of interbedded fine- to coarse-grained marine
sandstone and conglomerate but grades into non-marine sediments in southwestern Alberta.
In much of southeastern Alberta and southwestern Saskatchewan, the Viking Formation
consists of multiple, upward-coarsening cycles. Elsewhere, it is a single sandstone
body only a few metres thick. Sandstone- and conglomerate-filled channels are present
in several areas such as the Sundance, Edson and Crystal oil pools of west-central
Alberta. Conglomerates occur as far east as the Dodsland-Hoosier area in Saskatchewan.

Distribution of the Upper Albian Paddy Member is restricted to that area defined
by the Peace River Arch in northeastern British Columbia and northwestern Alberta.
The Paddy Member is a heterolithic sandstone, siltstone and shale that thins eastward
from 90 m near the Rocky Mountain Foothills to less than 5 m in the Interior Plains
(Tp 70, R 27W5). The Paddy Member was deposited under brackish-water conditions in
the inner to outer reaches of a large estuarine system (Leckie and Singh, 1991).
Westward, sediment becomes increasingly fluvial in nature. Much of the Paddy Member
in northwestern Alberta was deposited within a broad, shallow valley, a few hundred
kilometres long and several tens of kilometres wide, which was cut into previously
deposited sediments of the Cadotte Member (Leckie et al. 1990). A thick sequence
of paleosols in the correlative Boulder Creek Formation from the Rocky Mountain Foothills
(Leckie et al., 1989) is related to the valley incision and subsequent sea level
rise.

The Flotten Lake Sand in west-central Saskatchewan and east-central Alberta north
of Township 50 is an interval of sandstone and shale approximately equivalent to
the Viking Formation (Simpson, 1982). Up to 21.3 m thick, it is a fine- to medium-grained
sandstone with minor shale interbeds. The Flotten Lake Sandstone thins and decreases
in grain size toward the southeast.

In Manitoba, the Newcastle Member consists of bioturbated, interbedded fine-grained
sandstones and shales up to 12 m thick (McNeil and Caldwell, 1981). In southeastern
Saskatchewan and southern Manitoba, the unit forms lobate sand bodies 12 to 23 m
thick. Sandstones in the upper portion of the Newcastle Sandstone are more sheet-like.

In northeastern Alberta and northwestern Saskatchewan, the St. Walburg Sand consists
of interbedded sandstone, siltstone and shale occurring below the Fish Scales Zone.
The sandstone is fine grained and highly quartzose, glauconitic and kaolinitic. It
is present only north of Township 46 and thickens northward to a maximum of 32.9
m (Simpson, 1982).

Westgate Member

The Upper Albian Westgate Member (McNeil and Caldwell, 1981) comprises shale above
the Newcastle and below the Belle Fourche members. The shale is about 20 m thick
and consists of dark mudstone with minor amounts of bioturbated silty shale and bentonite.
The top of the unit is drawn at the Base of Fish Scales Zone of the Belle Fourche
Member.

Fish Scales Zone

The Fish Scales Zone (Figs. 20.1 and 20.5, 20.6, 20.7, 20.8, 20.9, 20.10)
is a basin-wide marker that demarcates the Albian/Cenomanian boundary (Lower/Upper
Cretaceous). It contains abundant fish remains (scales and skeletal material) within
finely laminated, generally nonbioturbated sandstone and siltstone. Pebbles and nodular
phosphorites occur locally. The Fish Scales Zone is composite, and there may be at
least three such beds across the basin.

The Fish Scales Zone is characterized by high organic carbon contents and a low
concentration of benthic foraminifera, and is interpreted as representing deposition
under poorly oxygenated bottom conditions. In Saskatchewan and Manitoba, the Base
of Fish Scales Zone may represent a major hiatus with substages missing below and
above it (Caldwell et al., 1978; Bhattacharya and Posamentier, this volume, Chapter
25), whereas in western Alberta, strata above and below the marker unit appear to
be more conformable (Stelck and Armstrong, 1981).

The Fish Scales Zone ranges in thickness from less than 1.5 to 21 m and consists
of very fine- to fine-grained sandstone, siltstone and shale with abundant fish scales
and skeletal material. Chert granules and pebbles as well as nodular phosphorite
occur locally. North and Caldwell (1975) noted that the Textularia alcesensis and
most of the Verneuilinoides perplexus and Flabellammina gleddiei zones associated
with the Fish Scales Zone were missing and presumed eroded in central Saskatchewan.
The Fish Scales Zone is poorly defined in central Saskatchewan and is commonly difficult
to recognize on wireline logs. In Manitoba, the Fish Scales Zone occurs within the
lowermost 6 to 10 m of the Belle Fourche Member (Late Albian to Middle Cenomanian).
Farther west, and in the northern United States, the Fish Scale Marker Bed occurs
near the top of the Mowry Shale. Also within the Belle Fourche Member (Fig.
20.1) and above the Fish Scales Zone, the Ostrea beloiti limestone beds represent
a distinctive marker consisting of inoceramid or ostreid fragments (McNeil and Caldwell,
1981) which, with an associated bentonite, can be traced as far south as Oklahoma
and westward into Montana. McNeil and Caldwell (1981) considered this bentonite to
correspond to the "X" bentonite in the United States. A disconformity separates
the Belle Fourche Member from the overlying Keld Member of the Favel Formation (McNeil
and Caldwell, 1981).

Barons Sandstone

In southwestern Alberta, the Cenomanian Barons Sandstone overlies the organic-rich,
radioactive shales of the Fish Scales Zone (Fig. 20.1).
The Barons Sandstone includes a series of isolated pods of sandstone and conglomerate
up to 7 m thick, 3 to 5 km wide and 5 to 15 km long. The sandstones thicken westward
and become more continuous toward the Rocky Mountain Foothills.

Big River Formation

In Saskatchewan, the term Big River Formation was assigned by Simpson (1982) to
the interval between the top of the Viking Formation or Flotten Lake Sand and the
base of the Second White Speckled Shale. The formation varies from 42.7 to 150 m
in thickness. It is predominantly shale, with minor amounts of fine- to medium-grained
sandstone, pelecypod and fish debris, thin chert-pebble layers, bentonite and phosphatic
sandstone. The Fish Scales Zone and the St. Walburg Sandstone occur within the Big
River Formation.

Upper Albian Outcrop

In the Rocky Mountain Foothills of southern Alberta, deposits equivalent to the
subsurface Joli Fou, Viking and lowermost Colorado shales up to the Fish Scales Marker
Bed are not recognized. The lowest sediments of the Blackstone Formation appear to
have been deposited diachronously. In southwestern Alberta, the lowermost Blackstone
Formation falls within the late Cenomanian Dunveganoceras Zone (Stott, 1963) whereas
northward, it lies within the Late Albian Neogastroplites Zone.

The Crowsnest Formation in southwestern Alberta is considered a stratigraphic
unit within the Blairmore Group. However, the formation may be, in part, correlative
with the Joli Fou and Viking formations of the Colorado Group (Fig.
20.1). The age of volcanic agglomerates making up the Crowsnest Formation is
poorly constrained but is generally considered to be Late Albian. Source areas or
pipes for the volcanic detritus are thought to be in the Ma Butte and Coleman areas,
where maximum sediment thicknesses of up to 484 m also occur.

Dunvegan Formation

The middle/upper Cenomanian Dunvegan Formation represents a southeastward thinning
and fining, fluviodeltaic wedge deposited above the Shaftesbury Formation (Fig.
20.6; Stott, 1982). The formation is recognized in northwestern Alberta, northeastern
British Columbia and southeastern Yukon Territory and reaches up to 380 m in thickness,
thinning eastward and southward. Outcrops in northeastern British Columbia and southern
Yukon Territory are of conglomerates and coarse-grained sandstones deposited in a
braided-river and alluvial-plain setting. In northwestern Alberta, at least ten sand-rich,
progradational cycles separated by regional transgressive surfaces represent the
southeastward advance of the Dunvegan Formation (Bhattacharya, 1988; Bhattacharya
and Walker, 1991). The progradation, though attributed to global lowering of sea
level at 94 Ma (Bhattacharya, 1988), also coincides with a major uplift in the Omineca
and Intermontane belts (Stott, 1982). The Dunvegan has been recognized as far south
as the Athabasca River (Stott, 1963). Farther south, the Dunvegan Formation is replaced
by marine siltstone and shale beds of the Sunkay Member of the Blackstone Formation,
where sedimentation was dominated largely by pelagic deposition. The Dunvegan Formation
is described in considerable detail by Bhattacharya (this volume, Chapter 22).

Kaskapau Formation

The Kaskapau Formation in northwestern Alberta is a predominantly shale succession
250 to 860 m thick lying between the Dunvegan and Cardium formations
(Figs. 20.1, 20.9).
Directly above the Dunvegan Formation, the Cenomanian
to Turonian Kaskapau Formation contains a series of northeast-trending, shingled
and backstepping, shallow-marine sandstone bodies encased in marine shale (Wallace-Dudley
and Leckie, 1993, in press). These are, in ascending order, the Doe Creek, Pouce
Coupe and Howard Creek members, which were deposited in a generally retrogradational
pattern, following the regional transgression of the Dunvegan Formation
(Fig.
20.19). The Doe Creek Member consists of several discrete, 0.5 to 7 m thick,
sandstone bodies which are 21 to 37 km long and 5 to 7 km wide. The Howard Creek
Member underlies the organic-rich, radioactive shales of the Second White Speckled
Shale, which likely was the source for hydrocarbons in the Doe Creek Member. The
Tuskoola and Wartenbe sandstones are poorly understood units within the Kaskapau
Formation, occurring above the Vimy Member in outcrop in northeastern British Columbia
(Stott, 1967). In southwestern Saskatchewan and southeastern Alberta, the thin, shallow-marine
Phillips Sandstone/Second White Speckled Sandstone may be correlative with the sandstones
of the lower Kaskapau Formation. The Phillips Sandstone occurs about 6 m below the
top of the Second White Speckled Shale and can be up to 38 m thick. Geographically,
the sandstone coincides with the location of the Sweetgrass Arch
(Fig. 20.3).

Second White Speckled Shale/Favel Formation

The Second White Speckled Shale is a basin-wide marker named from early drillers'
reports of abundant white specks in the shale, now known to be sand-sized fecal pellets
comprising coccoliths and coccospheres, concentrated by currents. The shale is characterized
by a high total organic carbon content, high hydrogen indices and high radioactivity
on well logs. Other elements include rare, thin, coarse- to very coarse-grained sandstone
beds, ammonites, peleycpods (Inoceramus), bentonites, pyrite, calcite concretions
and fish debris. On gamma-ray logs the Second White Speckled Shale interval is typically
radioactive as a result of elevated uranium contents associated with abundant kerogen
in the shale. Discrete radioactive spikes also occur as a result of bentonites deposited
in the shales.

The Second White Speckled Shale is the northern correlative of part of the Greenhorn
Formation in the United States and appears to correspond to a global anoxic event
and maximum sea-level rise that occurred near the end of the Cenomanian (Kauffman,
1977). In the western regions of the basin, adjacent to the Rocky Mountain Foothills
where the source rock is mature, considerable volumes of liquid hydrocarbons have
been generated. In eastern Alberta, Saskatchewan and Manitoba, sandy units in the
Second White Speckled Shale form an important reservoir for sweet, dry, biogenic
methane.

The calcareous, non-concretionary shale making up the Vimy Member of the Blackstone
and the middle Kaskapau formations in northern Alberta and British Columbia is, in
part, correlative with the widespread Second White Speckled Shale.

In Manitoba, the equivalent Favel Formation consists of highly fossiliferous,
chalk-speckled shale, argillaceous limestone, calcarenite and bentonite beds, ranging
from 11 to 46 m thick. The Keld Member of the Favel Formation is of latest Cenomanian
and Turonian age (McNeil and Caldwell, 1981) and is correlative with the Second White
Speckled Shale and with the Greenhorn Limestone in Colorado, Kansas, Wyoming and
South Dakota. The overlying Assiniboine Member is predominantly black, calcareous,
chalk-speckled shale with thin bentonite and calcarenite interbeds. The Favel Formation
may have been erosionally truncated in parts of eastern Saskatchewan and western
Manitoba (McNeil and Caldwell, 1981).

In the west, the upper Turonian is marked by a regressive event capped by an erosional
disconformity which can be traced across the basin. In Alberta, this unconformity
is marked by the conglomerate-veneered E5 surface of the Cardium Formation, whereas
in Manitoba, it lies between the Morden and Niobrara formations (McNeil and Caldwell,
1981). The unconformity approximately coincides with the 90 Ma eustatic lowstand
of Haq et al. (1987). Plint and Walker (1987) attributed Cardium shoreline progradation
and the development of unconformities within the formation as having a tectonic component
at the western margin of the basin as well.

Cardium Formation

The Turonian/Coniacian Cardium Formation of the Smoky Group in northwestern Alberta,
Rocky Mountain Foothills and Interior Plains is from 15 to 125 m thick and consists
of marine siltstone, sandstone and conglomerate, locally overlain by non-marine sediments.
The Cardium progradational wedge is restricted to northwestern and west-central Alberta
and northeastern British Columbia (Figs. 20.6, 20.10).
The formation generally thins eastward and southward, grading into shales of the
Colorado Group. The depositional history of the Cardium Formation is complex, with
six upward-coarsening cycles capped by erosional surfaces (Plint et al., 1988). The
Cardium shoreline trended northwest-southeast and migrated eastward in northeastern
Alberta. The formation is described in considerable detail by Krause et al. (this
volume, Chapter 23).

The Cardium Formation and its correlative units are disconformably overlain by
the Coniacian to early Campanian marine shales and sandstone of the upper Colorado
Group (Wapiabi and Niobrara formations).

Morden Formation

In Manitoba and eastern Saskatchewan, the Morden Formation, which is partly correlative
with the Cardium and upper Blackstone formations (Fig. 20.1)
is a zeolite-bearing, dark gray to black, noncalcareous shale with rare, thin bentonites.
Thicknesses vary from 3.5 to 55 m and the unit thins northwestward from the Manitoba
Escarpment. The formation is absent in central Saskatchewan (North and Caldwell,
1975).

Muskiki, Marshybank and Bad Heart Formations

The Muskiki Formation (Figs. 20.18 left pane, 20.18 right pane, 20.21)
overlies the Cardium Formation, and comprises rusty-weathering shales, conformably
overlain by siltstone and sandstone of the Marshybank Formation. A regional unconformity
marks the top of the Marshybank Formation and separates this from the younger Bad
Heart Formation of the plains (Plint, 1990; Plint et al., 1990). The Marshybank and
Bad Heart formations are overlain by the Wapiabi and Puskwaskau formations, respectively,
a predominantly shale succession with minor amounts of sandstone, which ranges in
thickness from 114 to 360 m. Members of the Puskwaskau Formation include the Dowling,
Thistle, Hanson, Chungo and Nomad members.

The type locality of the Bad Heart Formation was defined by McLearn (1919) at
the junction of the Smoky and Bad Heart rivers (in the northern plains of Alberta; Fig. 20.20). McLearn (1919) described the Bad Heart sandstone
as "10 - 25 feet of coarse sandstone, weathering reddish brown". Plint
and Walker (1987) documented a regional unconformity at the top of the Marshybank
(then termed the 'Bad Heart') which extended several hundreds of kilometres into
the basin, and marked a period of erosion in response to relative sea-level fall.
This unconformity also marked the base of the Bad Heart Formation in the Alberta
Plains (Plint et al. 1990).

The Bad Heart Formation has been redefined (Plint et al., 1990) as a unit comprising
fine-grained, silty sandstones containing abundant ooliths and areally restricted
to the Alberta Plains. The older portion has been reassigned to the Marshybank Formation,
which comprises non-oolitic, marine shelf and coastal plain sandstones and siltstones
in the foothills and adjacent subsurface. It includes rocks formerly included in
the Bad Heart Formation.

Figure 20.21 shows a detailed cross section extending
from the foothills of British Columbia eastward into the plains of Alberta, which
illustrates the unconformity. The cross section shows the dramatic thinning of the
Marshybank Formation in a basinward direction, as a result of erosional truncation,
and the eventual disappearance of sandstone (e.g., in 3-30-67-26W5). The Mistanusk
Creek outcrop (Fig. 20.20) is the principal reference
section for the Marshybank Formation and can be tied to the subsurface using a nearby
well (Fig. 20.22). The Marshybank comprises upward-coarsening
marine sequences that grade into a series of fine-grained, hummocky and swaley crossbedded
and parallel-bedded shoreline sandstones, commonly overlain by coastal plain coals
and fluvial units. The Marshybank paleogeography is characterized by northeast- to
southwest-trending shorelines (Fig. 20.24).

Farther basinward, sandstones of the Bad Heart Formation abruptly appear east
of a northwest-trending line. The top of the Marshybank Formation is marked by a
thin veneer of chert pebbles; this surface can be traced through the subsurface to
the base of the Bad Heart Formation (Fig. 20.21). Detailed
allostratigraphic outcrop and subsurface correlation of the Muskiki and Marshybank
formations are summarized in Figure 20.23. Plint and
Norris (1991) presented a detailed facies and paleogeographic analysis of the Muskiki
and Marshybank formations. The Marshybank is made up of a series of upward-coarsening,
dominantly mudstone and siltstone parasequences, many of which are capped by a pebble
bed or an erosive-based shoreface sandbody. Integration of outcrop paleocurrent data
with log-determined facies distributions allows former shoreline trends to be determined
(Fig. 20.24).

First White Speckled Shale/Niobrara Formation

The First White Speckled Shale is the informal name for a calcareous mudstone
with subordinate amounts of bentonite, fish remains, nodular phosphate, and concretions
of siderite and calcite. It is the upper of two white-speckled units that occur across
the whole basin (Figs. 20.1, 20.5, 20.6, 20.7, 20.8, 20.9, 20.10). The thickness
of the First White Speckled Shale is highly variable, ranging from 6 to 157 m. In
Saskatchewan, the First White Speckled Shale lies directly and unconformably on the
Second White Speckled Shale. The First White Speckled Shale is represented in part
by the Thistle Member of the Wapiabi Formation in the Rocky Mountain Foothills and
by the Niobrara Formation in the Manitoba Escarpment and southeastern Saskatchewan.
The First White Speckled Shale occurs within the Labiche Formation in northeastern
Alberta, in the Puskwaskau Formation in northwestern Alberta and in southern Alberta,
within the Colorado Group.

On gamma-ray logs, the First White Speckled Shale interval is typically radioactive
as a result of high uranium content associated with abundant kerogen. Discrete, radioactive
spikes also occur as a result of bentonites deposited in the shales.

The Niobrara Formation (latest Turonian to earliest Campanian in age) in Manitoba
is up to 73 m thick and consists of a lower calcareous shale succession overlain
by chalky shale. A local to regional unconformity exists between the Niobrara and
the overlying Pierre Formation; the break is less pronounced to the west (North and
Caldwell, 1975; McNeil and Caldwell, 1981; McNeil, 1984). The three lowest members
(Gammon Ferruginous Member, Pembina and Millwood members) of the Pierre Formation
are correlative with the uppermost Alberta Group to the west. The Gammon Ferruginous
Shale is a bentonitic shale ranging from 5 to 50 m in thickness. The Pembina Member
consists of 3 to 15 m of carbonaceous shale containing abundant bentonite beds overlain
by a non-carbonaceous succession. The late Campanian Millwood Member consists of
30 to 150 m of shale containing abundant fossiliferous and nonfossiliferous calcareous
concretions.

The Santonian Medicine Hat Formation (Warren, 1985), which occurs in southeastern
Alberta and southwestern Saskatchewan below the First White Speckled Shale, consists
of at least three upward-coarsening, very fine-grained sandstone and siltstone successions,
3 to 11 m thick, deposited in a shallow-marine shelf setting (Gilboy, 1987; Hankel
et al., 1989). The formation is up to 60 m thick. The Medicine Hat Formation
(Fig. 20.18d) forms a shallow-gas reservoir, the largest
and oldest gas field in Canada, containing locally derived biogenic gas.

Milk River/Chungo

The early Campanian Milk River Formation (Fig. 20.1)
is a sandy clastic wedge confined to the plains of southern Alberta and Saskatchewan
(Fig. 20.25) and the southern and central Rocky Mountain
Foothills. The Milk River Formation occurs, in part, within the Alberta Group and
within the Montana Group, but is not considered part of the Colorado Group. Sediments
of the Milk River Formation are exposed in southern Alberta along the Milk River,
as a result of uplift on the Sweetgrass Arch, but dip into the subsurface farther
north where the formation passes into shales, siltstones, and sandstones of the Alderson
Member of the Lea Park Formation (Fig. 20.26). The Alderson
Member contains nearly 150 billion m3 of recoverable gas reserves in the
"Milk River" gas pool (Meijer Drees and Myhr, 1981). It is dated at late
Santonian to early Campanian in age (Sweet and Braman, 1990). In the foothills, the
equivalent rocks are named the Chungo Member of the Wapiabi Formation. The older
term Chinook sandstone has been used in the central foothills (Gleddie, 1949). In
the southwest corner of Alberta the Nomad cannot be recognized and the Chungo has
been included within the overlying Belly River Group (Stott, 1963; Dawson et al.,
this volume,Chapter 24).

The Milk River Formation is thought to be disconformably overlain by marine shales
of the Pakowki Formation (Braman and Hills, 1990), the contact marked by a layer
of chert pebbles. The disconformity can also be recognized in the foothills, where
age-equivalent Nomad Member shales of the Wapiabi Formation overlie the Chungo Member
(Sweet and Braman, 1990).

At Writing-On-Stone Provincial Park (Fig. 20.25),
the Milk River Formation is about 100 m thick (Fig. 20.26)
and has been subdivided into three members (Meijer Drees and Myhr, 1981). The lowermost
Telegraph Creek Member comprises interbedded shales and sandstones that overlie the
First White Speckled Shale. The Telegraph Creek grades up into the massive cliff-forming
sandstone of the Virgelle Member. The Virgelle includes a lower unit interpreted
as a storm-dominated shoreface sandstone (McCrory and Walker, 1986) and an upper
unit, which has been interpreted as a tidal inlet complex (Cheel and Leckie, 1990).
Paleocurrent data and general mapping indicates that paleoshorelines were oriented
approximately east-west (Fig. 20.25). The overlying Deadhorse
Coulee Member is a heterolithic coal-bearing unit (Fig. 20.26)
interpreted as being of non-marine origin. The Milk River Formation passes basinward
into bioturbated sandy mudstones of the subsurface Alderson Member, which is equivalent
to the Hanson Member in outcrop.

Chungo Member outcrops have been described by Rosenthal et al. (1984) and Rosenthal
and Walker (1987). A modified correlation of these outcrops is shown in Figure 20.27.
The Chungo is interpreted as comprising a set of offlapping, shingled,
coarsening-upward units (i.e., parasequences) capped by a widespread erosion surface
marked by chert pebbles. In southwestern Alberta, the Chungo shoreface deposits interfinger
southwestward with Chungo non-marine sediments and pass northward into the mudstones
of the Hanson and Thistle members. Rosenthal et al. (1984) showed a similar relation
with Chungo-equivalent Hanson Member mudstones farther north.

Meijer Drees and Myhr (1981) demonstrated that, in the subsurface
(Fig. 20.26),
parasequences in the equivalent Milk River Formation similarly downlap
to the northeast onto a widespread, radioactive log marker coinciding with the top
of the First White Speckled Shale, which probably represents a condensed section.

Re-evaluation of the age of the Chungo Member and its lithostratigraphic equivalents
shows that it becomes younger to the northeast (Wall and Germundsun, 1963; Sweet
and Braman, 1989, 1990). The unconformity between the Milk River/Chungo and the overlying
marine shales of the Nomad/Pakowki, documented by Sweet and Braman (1990), suggests
a different relation at the upper boundary than that indicated by Rosenthal and Walker
(1987). The contact between Chungo and Nomad is sharp and unconformable (as suggested
by Rosenthal et al., 1984), rather than interfingering. In the subsurface, this unconformity
is shown as a prominent "shoulder" on sonic and resistivity well logs and
has been used as a major stratigraphic datum for mapping purposes (Figs. 20.5, 20.6, 20.7, 20.8, 20.9, 20.10, 20.18 left pane, 20.18 right pane).

The duration and significance of the basal Pakowki unconformity apparently decreases
northeastward, with sandy mudstones of the Alderson Member in Saskatchewan being
late early to late Campanian in age (i.e., Pakowki equivalent, Braman, pers. comm.),
rather than early Campanian. This is also supported by the offlapping geometries
depicted in Figures 20.26 and 20.27.

Isopach Maps

Isopach maps (Figs. 20.11, 20.12, 20.13, 20.14, 20.15, 20.16, 20.17)
were constructed over stratigraphic intervals
that were well constrained by consistent markers (maps of the Viking, Dunvegan and
Cardium formations are shown in Chapters 21, 22 and 23, respectively). The maps show
that over relatively short intervals of time, zones of maximum sediment accumulation
shifted across the basin.

Maximum sediment thickness from the Base of Fish Scales to the top of the Viking
Formation (Fig. 20.11) is found in northeastern British
Columbia, thinning regularly from 400 to 60 m into northwestern Alberta. Throughout
most of Alberta, the interval is 20 to 60 m thick. The region of thickest sediment
approximately coincides with the position of Peace River Arch. The thinnest sediments
occur adjacent to the Rocky Mountain Foothills in southwest Alberta where the Fish
Scales Zone laps onto sediments of the Blairmore Group or was not deposited. The
interval from the top of the Dunvegan Formation to the Base of Fish Scales
(Fig. 22.3, this volume) is thickest (>>400 m) in northeastern British Columbia
over the Peace River Arch and thins southeastward to 80 m in northwestern Alberta.

The isopach of Cardium to Second White Speckled Shale marker (Fig.
20.12) shows maximum thicknesses northwest of the Peace River Arch. A general
westward thickening trend is marred by a few, isolated sections locally thicker.
The interval from the First White Speckled Shale marker to the top of the Cardium
Formation (Fig. 20.13) shows a general thickening to
the southwest from 140 to 240 m. The uppermost isopach interval, from the top of
the Milk River Formation to the First White Speckled Shale marker (Fig.
20.14) shows two depocentres: in the southwest, isopach values increase to a
maximum of 150 m, whereas in the northwest, thicknesses of 110 m are attained.

Alternative isopach intervals, that include the entire Colorado Group of Alberta
(Figs. 20.15, 20.16, 20.17),
show regionally significant changes in sediment accumulation patterns for the period
from the Albian to the Santonian, characterized by a shift in depositional strike
from northeast-southwest to northwest-southeast.

Lithology of the Colorado Group Shales

The Lower Colorado Group shales, exclusive of the Joli Fou, form a wedge of marine
sediments comprising dominantly mudstone and claystone (Fig.
20.28) with subordinate amounts of siltstone and fine-grained sandstone. Bentonites
are common and increase toward the southwest. Sand and silt contents increase west-northwestward,
reflecting the influence of more proximal prodelta and shelf environments. The shale
is generally composed, in order of decreasing abundance, of mixed-layer illite/smectite,
quartz, kaolinite, potassium feldspar, siderite and pyrite with minor muscovite,
chlorite and biotite. The illite content of the mixed-layer clay increases to the
west with increasing burial depth and diagenesis.

The basal beds of the Fish Scales Zone are similar in composition to the adjacent
shales but contain coarser clastic detritus. Locally, total organic carbon (TOC)
and Hydrogen Index (HI) values may be up to 6 wt percent and 400 mgHC/gOC, respectively.

The Second White Speckled Shale contains abundant marine (Type II) organic matter
and is mineralogically distinct from the underlying Colorado Group shales. The Second
White Speckled Shale is a calcareous mudstone to claystone containing up to approximately
38 wt percent calcite and/or dolomite with abundant pyrite (3 to 6 percent) and organic
matter (4 to 11 wt percent). The calcite is largely bioclastic in origin, comprising
Inoceramus, planktonic foraminiferal and nannofossil remains. The "white specks"
are fecal pellets composed of nannofossil fragments. The silicate mineralogy is similar
to that of adjacent Colorado Group shales.

Geological History

The Colorado Group represents sedimentation within the Western Canada Foreland
Basin during a period when global sea level was generally high and rising, but interspersed
with major higher frequency sea-level falls. Sedimentation took place within an active
foreland basin, adjacent to a tectonically active hinterland. The erosional and depositional
events preserved within the Colorado Group reflect this intermix of tectonic and
eustatic controls.

The lowermost deposits of the Colorado Group are the Spinney Hill and Basal Colorado
sandstones. These are related to the initial marine transgression of the Colorado
Group over the Mannville Group. The overlying Joli Fou Formation contains marine
faunal evidence of the first connection between the Gulf of Mexico and the colder
waters of the Boreal seas from the Arctic. As such, the Joli Fou Formation represents
the first Cretaceous seaway to extend the entire length of the Western Interior.
The seaway may have been subsequently closed for part of Lower Colorado deposition
and possibly even landlocked when the connections to the Gulf of Mexico and Boreal
seas were closed (Williams and Stelck, 1975).

The coarser clastics of the Viking Formation are generally considered to be of
shallow-marine origin and, in places, tidally influenced. Several sea-level lowstands
may be represented in the Viking Formation, the major one dated at about 97 Ma. During
these sea-level lowstands, incised valleys were cut and subsequently infilled with
estuarine sediments during sea-level rise. One of the valleys now contains thick
conglomerates of the Crystal Oil Field (Reinson et al., 1988). Southwest winds dispersed
volcanic ash (bentonites) within the Joli Fou and Viking formations across southern
Alberta (Amajor, 1985). In northern Alberta, a sea-level lowstand resulted in an
incised valley system several hundred kilometres long, cut into the Middle Albian
Cadotte Member, from the Rocky Mountain Foothills to the Interior Plains. This incised
valley system is coeval with multiple paleosols in the Boulder Creek Formation in
northern British Columbia (Leckie et al., 1989) and in the Mill Creek/Bow Island
formations in southern Alberta. These paleosols formed when sedimentation rates on
the floodplains decreased in the more westerly portions of the basin during lowstands.
The subsequent sea-level rise deposited the estuarine, shallow-bay and shoreline
deposits of the Paddy Member.

The volcanoes that produced the Upper Albian(?) Crowsnest Formation in southwest
Alberta were surrounded by an inland flood plain and were probably of high relief.
The magma chamber may have been a hydrous alkaline trachyte (Pearce, 1970) that resulted
from crustal melting at about 25 to 35 km depth. Regional subsurface correlations
show that the overlying Fish Scales Zone pinches out westward toward the Crowsnest
Pass area (Fig. 20.7).

The shales of the uppermost Albian Shaftesbury Formation in northern Alberta and
British Columbia represent sea-level rise and a major basinwide transgression above
the Viking Formation and equivalent units. During this time, the Boreal sea connected
for a second time with the northward-advancing seas from the Gulf of Mexico. The
organic-rich Fish Scales Zone may represent the joining of the two seas. The Fish
Scales Zone is generally considered to contain a condensed section deposited during
a peak transgression of the Cretaceous Interior Seaway. It also marks the Albian/Cenomanian
boundary. In southern Alberta, the oldest record of the transgression is the Cenomanian
Blackstone Formation where it onlaps the Blairmore Group.

The Dunvegan Formation represents a major progradational event and a provenance
in the Yukon Territory and northern British Columbia. The Dunvegan Formation consists
of numerous stacked depositional cycles that prograded as wave, fluvial and mixed-influence
deltas (Bhattacharya and Walker, 1991). The progradation, though attributed to global
lowering of sea level at 94 Ma, also coincided with a major uplift in the Omineca
and Intermontane belts.

The sea-level rise that began in the Late Albian and reached its peak during the
early Turonian is inferred to be eustatic (Haq et al., 1987; Caldwell, 1984). It
resulted in basin-wide deposition of the coccolithic Second White Speckled Shale
(Vimy Member) within which the Cenomanian-Turonian boundary occurs (Stelck and Wall,
1954). The Second White Speckled Shale is represented by the argillaceous, highly
calcareous Favel Formation in Manitoba (McNeil and Caldwell, 1981). The calcium carbonate
content of this interval generally increases eastward away from the Cordillera.

During the Turonian, a major regression, in large part related to eustatic fall,
resulted in a basin-wide disconformity and deposition of the non-marine to shelf
sediments of the Cardium Formation. High-frequency fluctuations in sea level resulted
in the complex depositional patterns now preserved.

The peak of the marine transgression following Cardium sedimentation is represented
by the First White Speckled Shale. Planktonic faunas in this unit indicate a warm-temperate
climate in at least the eastern part of the basin during the latest Cenomanian, Turonian
and early Santonian to earliest Campanian (McNeil, 1984). During maximum marine transgressions,
warm waters from the Gulf of Mexico possibly extended as far north as 54° N
latitude, increasing water temperatures by up to 5° C to a temperature near
20° C (McNeil and Caldwell, 1981). In the First and Second White Specks and
perhaps the Fish Scales Zone, the presence of biogenic chalk and planktonic foraminifera
indicate open-marine conditions within the seaway during the peaks of the marine
transgressions.

The final regressive event of the Colorado Group began during early Campanian
time, and was culminated by extensive fine- to medium-grained shoreline sandstones
of the overlying Virgelle Member (Milk River Formation) and the Chungo Member (Wapiabi
Formation) which extend from southeastern Alberta and northern Montana to the central
Alberta Foothills. The shoreline prograded as a sheet of wave-dominated sandstone
up to 59 m thick; it extended along strike for at least 350 km. The Virgelle and
Chungo members exposed in outcrop grade laterally northward into interbedded sandstone,
siltstone and shale of the Alderson Member (of the Lea Park Formation) in the subsurface
(Miejer Drees and Myhr, 1981; Rosenthal and Walker, 1987). In west-central Alberta,
the shoreface sandstone of the Chinook Member is slightly younger than the Chungo
Member, although Stott (1967) included the Chinook within the Chungo Member. The
influence of tides in the foreland basin at this time is recorded in a tidal-inlet
sequence preserved in outcrop at Writing-On-Stone Park in southern Alberta (Cheel
and Leckie, 1990). During early to early late Campanian time, sea level rose again
and the marine shales of the Pakowki Member/Nomad Member were deposited.

McNeil, D.H. 1984. The eastern facies of the Cretaceous System in the Canadian
Western Interior. In: The Mesozoic of Middle North America. D.F. Stott and D.J. Glass
(eds.). Canadian Society of Petroleum Geologists, Memoir 9, p. 145-171.

North, B.R. and Caldwell, W.G.E. 1975. Foraminifera from the Late Cretaceous
System of Saskatchewan. In: The Cretaceous System in the Western Interior of North
America. W.G.E. Caldwell (ed.). Geological Association of Canada, Special Paper 13, p. 303-331.

Plint, A.G. 1990. An allostratigraphic correlation of the Muskiki and Marshybank
formations (Coniacian-Santonian) in the Foothills and subsurface of the Alberta Basin.
Bulletin of Canadian Petroleum Geology, v. 38, p. 288-306.

Stott, D.F. 1982. Lower Cretaceous Fort St. John Group and Upper Cretaceous Dunvegan
Formation of the Foothills and Plains of Alberta, British Columbia, District of Mackenzie
and Yukon Territory. Geological Survey of Canada, Bulletin 328, 124 p.

Williams, G.D. and Stelck, C.R. 1975. Speculations on the Cretaceous palaeogeography
of North America. In: The Cretaceous System in the Western Interior of North America.
W.G.E. Caldwell (ed.). Geological Association of Canada, Special Paper 13, p. 1-20.